The present invention relates in general to simultaneous signal attenuation measurement systems and, in particular, to the use of code division multiplexing in such systems to identify attenuation characteristics associated with individual signal components.
Signal attenuation measurements generally involve transmitting a signal towards or through a medium under analysis, detecting the signal transmitted through or reflected by the medium and computing a parameter value for the medium based on attenuation of the signal by the medium. In simultaneous signal attenuation measurement systems, multiple signals are simultaneously transmitted (i.e., two or more signals are transmitted during at least one measurement interval) to the medium and detected in order to obtain information regarding the medium.
Such attenuation measurement systems are used in various applications in various industries. For example, in the medical or health care field, optical (i.e., visible spectrum or other wavelength) signals are utilized to monitor the composition of respiratory and anesthetic gases, and to analyze tissue or a blood sample with regard to oxygen saturation, analyte values (e.g., related to certain hemoglobins) or other composition related values.
The case of pulse oximetry is illustrative. Pulse oximeters determine an oxygen saturation level of a patient""s blood, or related analyte values, based on transmission/absorption characteristics of light transmitted through or reflected from the patient""s tissue. In particular, pulse oximeters generally include a probe for attaching to a patient""s appendage such as a finger, earlobe or nasal septum. The probe is used to transmit pulsed optical signals of at least two wavelengths, typically red and infrared, to the patient""s appendage. The transmitted signals are received by a detector that provides an analog electrical output signal representative of the received optical signals. By processing the electrical signal and analyzing signal values for each of the wavelengths at different portions of a patient pulse cycle, information can be obtained regarding blood oxygen saturation.
Such pulse oximeters generally include multiple sources (emitters) and one or more detectors. A modulation mechanism is generally used to allow the contribution of each source to the detector output to be determined. Conventional pulse oximeters generally employ time division multiplexing (TDM) signals. As noted above, the processing of the electrical signals involves separate consideration of the portions of the signal attributable to each of the sources. Such processing generally also involves consideration of a dark current present when neither source is in an xe2x80x9conxe2x80x9d state. In TDM oximeters, the sources are pulsed at different times separated by dark periods. Because the first source xe2x80x9conxe2x80x9d period, the second source xe2x80x9conxe2x80x9d period and dark periods occur at separate times, the associated signal portions can be easily distinguished for processing.
Alternatively, pulse oximeters may employ frequency division multiplexing (FDM) signals. In the case of FDM, each of the sources is pulsed at a different frequency resulting in detector signals that have multiple periodic components. Conventional signal processing components and techniques can be utilized to extract information about the different frequency components.
In order to accurately determine information regarding the subject, it is desirable to minimize noise in the detector signal. Such noise may arise from a variety of sources. For example, one source of noise relates to ambient light incident on the detector. Another source of noise is electronic noise generated by various oximeter components. Many significant sources of noise have a periodic component.
Various attempts to minimize the effects of such noise have been implemented in hardware or software. For example, various filtering techniques have been employed to filter from the detector signal frequency or wavelength components that are not of interest. However, because of the periodic nature of many sources of noise and the broad spectral effects of associated harmonics, the effectiveness of such filtering techniques is limited. In this regard, it is noted that both TDM signals and FDM signals are periodic in nature. Accordingly, it may be difficult for a filter to discriminate between signal components and noise components having a similar period.
The present invention is directed to a simultaneous signal attenuation measurement system employing code division multiplexing (CDM). The invention allows for analysis of a multiplexed signal to distinguish between two or more signal components thereof based on codes modulated into the signal components. The CDM codes are nonperiodic thereby facilitating various processing techniques for distinguishing the signals of interest from noise or other interference. Moreover, the invention allows for a variety of hardware and processing options that may reduce costs, simplify system operation and improved accuracy of the attenuation measurements.
According to one aspect of the present invention, codes are modulated into the transmitted signals of a signal attenuation measurement system. The system includes at least two signal sources (e.g., having different wavelengths) that are pulsed by source drives to a medium under analysis. One or more detectors receive the first and second signal from the medium (e.g., after transmission through or reflection from the medium) and output a composite signal reflecting contributions corresponding to each of the transmitted source signals. The detector signal is thus a multiplexed signal composed of at least two signal components. In accordance with the present invention, the source drives are operated to modulate each of the source signals based on a code. For example, each drive may pulse a corresponding one of the signal sources between a high output or xe2x80x9conxe2x80x9d state and a low value or xe2x80x9coffxe2x80x9d state. It will be appreciated that, depending on the sources employed, substantial photonic energy may be transmitted in the nominal xe2x80x9coffxe2x80x9d state. Accordingly, in the context of the source signals, a code may be conceptualized as a bit stream of xe2x80x9c0sxe2x80x9d and xe2x80x9c1sxe2x80x9d, where xe2x80x9c0xe2x80x9d corresponds to an off state, xe2x80x9c1xe2x80x9d corresponds to an on state, and the bit length corresponds to a base unit of time that generally reflects the shortest pulse length utilized in driving the sources.
The codes define source signals that have nonperiodic characteristics. That is, due to the codes, there is at least a component of each source signal that is not described by a regularly repeating temporal pattern. As will be understood from the description below, however, the codes themselves may be concatenated in the source signal and a periodic modulating signal may carry the coded signal.
A number of preferred characteristics have been identified for the codes. Among these are:
1. the codes for the different sources are preferably mathematically orthogonal;
2. the numbers of 1s and 0s in a code should be about the same;
3. the distribution of 1 s and 0s within a code should be fairly even; and
4. the distribution of transitions between 1s and 0s within a code should be fairly even.
These preferences and some bases therefor are described in detail below. The codes utilized in accordance with the present invention preferably have one or more of these characteristics and, more preferably, have all of the noted characteristics.
According to another aspect of the invention, a detector signal is processed in a signal attenuation measurement system to demultiplex the detector signal and extract component information therefrom based on nonperiodic codes. In particular, the detector signal is first processed to provide a processed signal for demultiplexing and the processed signal is then demultiplexed using at least one coded demultiplexing signal that includes a series of values defining a nonperiodic code. Information is thereby obtained regarding first and second signal components of the detector signal. This information can be utilized in an attenuation analysis to determine an attenuation related parameter of a medium under analysis.
The initial processing of the detector signal may include various processing steps and components depending on the specific application and implementation. For example, where the detector signal is an analog signal, initial processing may involve analog to digital conversion. Preferably, such conversion is implemented using a fast analog to digital converter that digitally samples the detector signal multiple times per source cycle. Such a convertor in combination with processing techniques enabled by code division multiplexing allows for improved measurement accuracy and hardware implementation options for certain attenuation measurement applications. The initial processing may further or alternatively include signal filtering to reduce undesired components, signal amplification including, e.g., DC rectification to remove or avoid amplifying DC or low frequency components especially in the case of DC coupled sources, and/or other signal enhancement processing.
Preferably, the demultiplexing process involves the use of a unique demultiplexing signal for each signal component of interest, e.g., corresponding to each signal source. In this regard, the same codes used for modulating the source signals may be used to demodulate the detector signal. However, for mathematical convenience, the demodulating codes may be conceptualized as a series of xe2x88x921s and +1s rather than 0s and 1s as discussed above in relation to the modulating codes. The coded demodulating signal may be filtered to compensate for certain wave shape distortions resulting from bandwidth limitations and non-linearities and/or to reduce response at certain frequencies. In addition, the codes may be pre-computed to reduce storage and processing requirements.